This invention relates generally to infrared (IR) detection systems and more particularly to such systems for which application requirements necessitate the use of non-axisymmetric optics systems.
Current IR detection systems comprise focal plane arrays of sensor elements that operate at cryogenic temperatures and accordingly require array mounting in a dewar or similarly cooled enclosure. It is also necessary to provide cold shielding of the detector element array, to avoid the degradation in signal-to-noise ratio that results when the detector element fields of view include optics system structural members that are at temperatures above cryogenic. Cold shielding currently is usually accomplished using one or more of the following basic methods:
(1) Place all or part of the focusing elements inside the cold region surrounding the focal plane detectors;
(2) Extend the dewar window in front of the focal plane and thereby move the aperture in the cold shield as far as possible from the detectors;
(3) Use reflectors to produce a "Narcissus" effect over the entire field of view; and
(4) Design the focus system to place its exit pupil at the cold shield aperture.
The last of the above-listed techniques derives from the so-called "Solar Coronagraph" developed some decades ago by B. Lyot for observation of the sun's corona. This instrument comprised a re-imaging optical system with an occulting disk disposed in the plane of the first image and a stop located at the exit pupil adjacent the plane of the second image. Similar optical configurations have later been used in telescopes and other applications in which the rejection of out-of-field radiation is critically important. In such applications Lyot's occulting disc is omitted and a field stop commonly substituted, but the exit pupil stop serves its original purpose and has come to be known as a "Lyot stop".
Lyot stops have found use in infrared imaging systems, where they are particularly useful in reducing the effects of radiation emitted from uncooled walls of the system which would otherwise be included within the IR sensor element's field of view and the radiation from which would thus reduce the signal-to-noise ratio of the system. The practical application of Lyot stops for cold shielding purposes in infrared detector systems is not without some attendant problems, principal among which is that their effectivity is critically dependent on the quality of the exit pupil and as hereinafter explained this can be very difficult to control.
For best performance of IR detection systems the power elements of the system need to be corrected for image quality at the surface of the detector array. The optics cannot at the same time be fully corrected for the effects of astigmatism on the aperture stop image at the exit pupil point, and as a result the uncorrected effects of this aberration significantly degrade exit pupil formation. Then because the performance of a Lyot stop depends on a well formed exit pupil, such uncorrected aberrations at the exit pupil result in reduced cold shield efficiency.
Poor efficiency can be a significant problem even in IR detection systems in which all the mirror, lens and other elements are symmetric with respect to the system optical axis. The problem is significantly more difficult in the case of systems in which these elements are non-axisymmetric, primarily because of the larger optical aberrations that are characteristic of such systems. Thus the reduction in cold shield efficiency attributable to exit pupil anomalies is particularly troublesome in systems having relatively large aberrations due to the presence of off-axis optical elements.
Non-axisymmetric optical element configurations may be necessary to accommodate a number of different detection system requirements, most commonly a requirement that a scanning device such as a rotating or nutating mirror be positioned directly on-axis. Such a system is illustrated in U.S. Pat. No. 4,558,222, for example, which is also of interest because it includes a re-imaging optical system with a stop located at a pupil coincident with the cold shield aperture.
The present invention is addressed to the problem of uncorrected aberrations in infrared sensor systems of this general kind, and specifically in such systems in which the problem of astigmatism is compounded by the inclusion of non-axisymmetric optical elements. The invention has as a primary objective the provision of such systems that provide high efficiency of cold shielding notwithstanding the quality of the exit pupil even as compromised by optical system asymmetries, and that provide this improvement with relatively little attendant additional system complexity or cost.